Solid oxide fuel cells (“SOFCs” below) operate at relatively high temperatures, using an oxide ion conducting solid electrolyte as an electrolyte, with electrodes placed on both sides thereof, and with fuel gas supplied to one side thereof and oxidizer (air, oxygen, or the like) supplied to the other side thereof.
In such SOFCs, steam or CO2 is produced by the reaction between oxygen ions passed through the oxide ion conducting solid electrolyte and fuel, thereby generating electrical and thermal energy. The electrical energy is extracted from the SOFC, where it is used for various electrical purposes. On the other hand, thermal energy is used to raise the temperature of the fuel, the reformer, the water, the oxidant, and the like.
It is known that fuel cell units degrade with usage over long time periods. Unexamined Patent Application 2007-87756 (Patent Document 1) sets forth a solid oxide fuel cell. It is stated that in this fuel cell, degradation of the fuel cell units can be reduced by adjusting the flow rate of fuel.
A fuel supply amount control device, fuel supply amount control method, and electrical power supply system are also set forth in Unexamined Patent Application 2003-217627 (Patent Document 2). In this fuel supply amount control device, the amount of fuel supplied is compensated when the electrical power extractable from a predetermined amount of fuel supplied decreases due to degradation of fuel cell units.
In general, the electrical power extractable from degraded fuel cell units is reduced relative to the same fuel supply amount. Therefore when the electrical power which can be extracted has declined, the fuel supply amount is compensated to further raise the temperature of fuel cell units through increased fuel supply amounts, thereby lengthening the time period over which an initial rated output power can be extracted from the fuel cell.